40 The American Geologist. January, 1905 
2. The variation in the specific gravity of the constit- 
uents ; 
3. The change of specific gravity which minerals ex- 
perience through weathering, decomposition and alteration. 
A 5-gram sample of the rock was found to work well 
in the size of separator used. After pouring in the solution, 
whose density was first adjusted to separate the feldspar group 
from the remaining heavy minerals, the weighed rock sample 
was added. The apparatus was thoroughly shaken and al- 
lowed to stand until a separation had taken place. The 
floating feldspars were then poured onto a filter, washed, 
dried and weighed. The residue was likewise removed and 
kept to be treated later. After adjustment of the density 
of the solution, the feldspars were parted from each other 
and weighed. The method of procedure in this and in sepa- 
rating quartz from the iron minerals was precisely as out- 
lined above. 
Three separate analyses of the granite were made. The 
average is tabulated in the summary near the close of the 
paper. 
GRAPHIC ANALYSIS 
The term "graphic analysis" is suggested by Dr. A. A. 
Julien* as a general heading to embrace all the various pro- 
cesses involved in estimating mineral composition in the thin 
section. Since the process of Delesse* was one depending on 
tracing the outlines of mineral grains on the polished hand 
specimen, this it would seem may also, legitimately be in- 
cluded under "graphic" methods. The method of Jevon, of 
direct comparison of the breadth of grains by applying a 
scale to a polished face, may also be mentioned here. 
(c). MEASUREMENT OF DIAMETER OF GRAINS. 
As Stated at the opening of this paper, there are three 
different processes which can be carried out by means of 
the microscope to obtain a basis for estimating the mineral 
composition of igneous rocks. First, the average diameters 
of the grains of the different minerals may be found by meas- 
uring the length of the cross section in thin slices of the 
♦Genesis of the Amphibole Schists and Serpentines of Manhattan 
Island, New York. Bull. Geo/. Soc, Amer., Vol. 14, p. 460. 
• Proced6 Mecanique pour determiner La Composition des Roches. 
Pans, 1866. Annales des Mines, (4), Vol. XIII., 1848, p. 379. Compte Rendu 
Vol. XXV, 1847, p. 544. 
